One of the main issues with cervical plating systems available today is that the plate is placed onto the vertebral body in a “free hand” technique where essentially the surgeon determines where the plate should go based on anatomical reference points such as the interbody device just placed to obtain a fusion, the inferior (caudal) endplate of the cephalad vertebral body, the superior endplate of the caudad vertebral body, the disc above the surgical site and the disc below the surgical site. The optimal placement of the plate and screw is as close to the inferior border of the superior vertebral body, the superior end plate of the inferior vertebral body, and as far as possible from the disc above and the disc below although there is some variation on surgeon preferences.
Proximity to the end plates allows for better bone quality and distance from the discs above and below allows for preventing damage to those discs. Thus, ideal or perfect plate placement can be very important to a successful operation.
The land marks mentioned above may be easily seen on a saw bone. However, during surgery the presence of limited exposure, presence of blood, limited visualization of the end plates due to plate design, and inability to see the disc above and below due to lack of sufficient visual variability makes free hand placement difficult.
In conventional procedure, temporary fixation pins are often used to allow for holding the plate in position while a more exact determination of the location and alignment of the plate is performed. However, these pins are often quite small in diameter allowing the plate to move around the pin. Also variation in bone anatomy and texture sometimes results in the pin “sliding” and not entering the bone of the vertebral body in the intended position thus compromising the position.
All these difficulties add to the complexity of an operation and make the result less reproducible from patient to patient and operation to operation.
There are cervical plate/cage designs that are available that mate the cage to the plate. However, these systems only work with one level procedures and cannot be used for multilevel designs if the surgeon chooses to use a multilevel or single level plate with two screws in each vertebral body.
A system according to the present invention allows for solving all the above mentioned problems. By providing a cage that has a screw insertion hole or fixation point at a fixed distance from its top edge, for example, a system according to the present invention gives the surgeon a fixed reference point based on which the plate can be referenced and permanent bone screw positions may be located and drilled. The cage can be manufactured so that the reference hole is a fixed distance from, for example, the top of the cage, which in turn makes it a fixed distance from the inferior endplate (or superior endplate depending on the orientation of the cage) regardless of the height of the cage. Because this distance is fixed, a system of either a cervical plate or a drill guide for any cervical plate that references off of the hole in the cage will allow for the perfect positioning of the cervical plate in relationship to discs above and below and the superior and inferior endplates every time. The reference point on the cage also allows for the usage of a very stable fixation mechanism that allows for easier control of the cervical plate before it is definitively fixed to a vertebra with a screw or screws. A version of the present invention will allow for adjustment of plate position in relation to the fixation point to allow for optimal plate fixation and position.
A system according to the present invention will also allow for measurement of length of a plate necessary via calipers. This task of length selection is another task that is presently done “free hand” by trial and error.
The cage/plate system can be used as a dedicated system that mate with each other or the cage can be used with any cervical plate system that has a drill guide specifically designed for attachment to a cage and positioning.
Alternatively specific plate holders can be designed that temporarily hold the plate to the cage and then removed after the plate is screwed definitively into position by screws into the vertebra.
A system according to the present invention can be used with a one level cervical plate or a multilevel cervical plate without modification.
The modification for the plate is the concept of adding a hole or connection mechanism on the cephalad top side of the cervical plate on at least one end or both ends of the plate that can mate to a cage configured to provide the necessary predetermined distance for an ideal location of mounting holes. This modification can be done to any plate with any locking mechanism.
The connection between the cervical plate and the cage is removed once the plate is attached to the vertebras with the definitive screws. A version can be envisioned that the connection is not removed.